The prior art provides many types of materials which filter water soluble contaminants as well as particulate matter. These filters, while fairly effective in the applications for which they were designed, do not reduce the contaminant level sufficiently. Such filters are expected to meet environmental guidelines, while having higher dirt holding capacity, lower pressure drop, lower cost and greater durability. They include those for point of entry (POE, for example, whole house or whole building), point of use (POU, for example, at a specific faucet), in a pitcher, or in a portable device. Pitchers and many portable devices require a low pressure drop, since flow must be achieved by a manually driven pump, by gravity, or by suction, as in a straw device. As the particulate reduction requirements increase, thicker filters become necessary, with commensurate increases in pressure drop.
The need for safe drinking water is growing rapidly, particularly in the developing world, Rapid population growth in China is stressing water supplies and there is little focus on improvement of municipal water systems, particularly in rural areas. For years locals have relied on the river to provide them with drinking water and to irrigate their crops. Now, though, many of those same people are dying of cancer. Many small hamlets have joined the ranks of what China's media calls the country's “cancer villages.”
The situation is graver in African communities, where most of the time there is no power or local sources of water, and drinking water is carried from heavily contaminated surface sources. The death rate from gastro-intestinal diseases is endemic throughout these areas, and millions, particularly infants, die from waterborne diseases.
Fibrous media is used to remove microbial pathogens. Activated carbon is capable of removing the bad taste and odor from water as well as chlorine and other reactive chemicals such as halogenated hydrocarbons and pesticides that can be carcinogenic. Ion exchange resins are used to remove metal and other ions from water. However, no single material or chemical exists that is able to remove all contaminants.
Granular activated carbon (GAC), useful for retaining many soluble contaminants, is difficult to design into useful filters, particularly where the device is portable and subject to vibration and motion. Loose particles migrate causing channeling and clogging of the bed. Carbon filter blocks are readily fouled by small particles, and a prefilter is necessary to protect them.
Non-woven fibrous media are used extensively for filtering water of both particulate and chemical contaminants. Such a structure minimizes channeling and also allows significant filter design variations. Non-wovens are manufactured by low cost assembly methods such as paper making and are used extensively in residential and commercial water purification devices.
While having approximately equivalent (BET) surface area and iodine number, powdered activated carbon (PAC) has higher external surface area than GAC and it is generally recognized that it would have superior adsorption kinetics than GAC. However, combining it into a non-woven matrix is difficult because adhesives are required to attach it to the fiber matrix. U.S. Pat. No. 5,759,394 states that “The very act of using an adhesive to hold the particles results in a portion of the surface of the powder particles being contaminated by the adhesive and therefore becoming ineffective for filtration. A balance has to be met between the strength of the immobilization versus the maintaining of effectiveness of the powder layer. In order to minimize this contamination, larger particles are often used so that the point of contact between the surface adhesive and powder particles is small. In typical gaseous applications using activated carbon the particles are frequently 100 microns and larger; and, finely powdered activated carbon is basically only used in liquid decolorization applications despite the fact that fine powder activated carbon holds the potential of much more rapid kinetics”.
Preferred filters would remove taste and odor-causing small molecules, cysts such as Cryptosporidium, bacteria, virus, turbidity (that might be toxic and might also harbor pathogens), chlorine and halogenated organics formed by chlorination of pathogens, and organic matter. The U.S. EPA Guide Standard provides criteria for mechanical filters for drinking water. Retention should be >6 LRV (log retention value) for bacteria, >4 LRV for virus and >3 LRV for cysts, while in a background of A2 fine test dust, an organic contaminant and under adverse conditions of salinity, pH and temperature.
Iodine has been used to sterilize drinking water. Problems arise because it is necessary to enrich the water with a large quantity of iodine (at least 1 mg/l). The iodine has to remain in contact with the water for a long time, followed by the subsequent removal of iodine from the outflowing water. At iodine concentrations in water exceeding 4 mg/l, water acquires a distinct iodine odor. Long term consumption of iodinated water affects the thyroid gland. Scavenging of iodine downstream of the filter is necessary to assure a concentration below 0.5 μg/l.
Given this, there is a need among consumers for drinking water filter devices with better adsorption efficiency for soluble contaminants, superior microbial and particulate retention and that also has a low pressure drop, and preferably a pressure drop that is low enough to be used with gravity feed.